1. Field of the Invention
The present invention relates to an emergency core cooling system (ECCS) of a nuclear power plant particularly of a boiling water reactor (BWR).
2. Related Art
The most recently known BWR is an advanced boiling water reactor (ABWR). The ECCS of the ABWR is divided into three sections. The safety of the ABWR of such type has been significantly improved compared with previously known ECCSs each including only two divided sections.
The outline of the ECCS of a known ABWR having divided three sections will be described hereunder with reference to FIGS. 6 and 7.
FIG. 6 is a schematic view of a front line of the ECCS of a known ABWR divided into three sections including first, second and third safety divisions. As illustrated in FIG. 6, each safety division of the ECCS includes a low-pressure flooding system (LPFL) 1, a residual heat removal system (RHR) 2, a reactor component cooling system (RCW) 3, not shown in FIG. 6, a reactor component sea water cooling system (RSW) 4, not shown in FIG. 6, and an emergency diesel generator (DG) 5.
A high-pressure core flooding system (HPCF) 8 is provided for the first and second safety divisions, and a reactor core isolation cooling system (RCIC) 7 is provided for the third safety division. For convenience, the components of the ECCS illustrated in FIG. 6 are referred to as the ‘front line’ of the ECCS. Each of the areas sectioned by a physical separation wall is referred to a ‘safety division’. The safety divisions are designed based on safety so as to isolate one area from another during an incident, such as fire or flooding, that might occur inside a nuclear power plant and threaten the safety of the nuclear power plant. By isolating the safety divisions from each other, even if such an incident occurs in one safety division, the other safety divisions can be kept unaffected.
FIG. 7 is a schematic view of a support line of the ECCS of a known ABWR illustrating a mechanism for cooling the heat generated in a nuclear reactor and a primary containment vessel.
As illustrated in FIG. 7, each of the three systems includes the RCW 3 and RSW 4, respectively, and the same reference numerals indicate the same components in each of the three systems. Each system includes a RHR heat exchanger (RHR Hx) 12, RCW pumps 14, RSW pumps 15, an emergency heat-ventilating and air-conditioning system (HVAC) and emergency reactor auxiliary components 21, an IA and CRD pumps 22, containment vessel internal components (reactor internal pump (RIP) and drywell cooler (DWC)) 23, normal auxiliary components 24, and an RCW loop (circulation pipes) 25.
In each system, the LPFL 1 and the RHR 2 share pumps to send water to the RHR Hx 12 by circulating the water in the reactor or in the suppression pool inside the primary containment vessel to cool the reactor and the primary containment vessel. The heat from the reactor and the primary containment vessel is transmitted to the RHR Hx 12 and is cooled at the RCW 3. Then, the heat transmitted to a RCW heat exchanger (RCW Hx) 13 is cooled by sea water.
Since, as mentioned above, the ECCS for cooling the reactor and the primary containment vessel of the ABWR is divided into three sections, the possibilities of accidents due to failure of cooling occurring are significantly reduced compared with other known ABWRs. Hereinafter, for the sake of convenience, the RCW 3 and the RSW 4 are referred to as the ‘support line’ of the ECCS.
However, the above-described reactor cooling system of the ABWR requires piping for each loop of the RCW 3 or, in other words, requires three sets of piping. The cost of the piping for each RCW 3 makes up the largest proportion of the entire cost of the ABWR. Thus, the cost of the above-described ABWR is no less than the cost for other previously known reactors.
In order to solve the above-mentioned problems or inconveniences, a semi-four-section ECCS has been provided. This semi-four-section ECCS comprises a two-loop reactor cooling system, wherein the front line is divided into four safety divisions, as illustrated in FIG. 8 (for example, refer to Japanese Unexamined Patent Laid-open Publication No. 2000-275380).
In this way, cost efficiency, operating rate, and safety are improved in comparison with a full-three-section ECCS for the known ABWR such as mentioned above.
The front line of the semi-four-section emergency core cooling system (ECCS) is divided into four sections. However, these four systems provided for the four sections of the front line are more systems than necessary. In addition, four emergency power supplies are required for the four systems. As a result, the ECCS becomes expensive and large in size.
Especially, in order to improve the safety of a next-generation BWR plant, a passive containment cooling system (PCCS) independent from the active ECCS is disposed so that the cooling ability and the reliability of the primary containment vessel are maintained even when the ECCS completely loses its functions. In this way, the next-generation BWR plant has achieved extremely advanced multiple-levels of protection.
Moreover, recently an innovative reactor containment vessel having both a double containment function and an air cooling function has been introduced. By employing this containment vessel, the safety of the next-generation BWR plant has been enhanced significantly. Even after the water source of the PCCS is exhausted, the containment vessel can be naturally cooled by outside air. The containment vessel is compact and stores active components and heat exchangers in a compartment located in the lower part of the primary containment vessel. However, a known active ECCS comprises a large number of components, which makes it difficult to arrange all the components inside the compact containment vessel.